With the advancements of electronic and wireless technologies, portable wireless devices such as cellular phones have become increasingly popular. New generation cellular phones integrate a great number of functionalities, such as gaming, personal data assistants (PDA), e-mail, digital cameras, general packet radio services (GPRS), global positioning systems (GPS), and Blue Tooth (BT). Further, incorporation of wireless local area network (WLAN) technology into smart phones appears inevitable. When integrating a WLAN radio into a compact multi-radio portable device, a design challenge is the reduction of transmit and receive interference between simultaneously operating radios.
Typically in WLAN radios two or more antennas are used to transmit and receive signals, wherein a different antenna is used for increasing the spatial diversity or utilizing a different portion of the radio spectrum. Therefore, in most WLAN radios double pole double throw (DPDT) switching is adopted for switching between the antennas. Spatial diversity provided by using multiple antennas increases probability of data recovery and avoids nulls caused by multi-path propagation. The distance between antennas is typically desirable to be less than 20% of a wavelength to ensure spatial diversity and reduce the effects of antenna coupling.
FIG. 1 shows a conventional DPDT T/R (transmit/receive) switch 140 for use in a WLAN radio utilizing spatial diversity. The DPDT switch comprises two switches that are simultaneously switched between different circuit paths. A first antenna 150 is connected to a first RF switch port 155. A second antenna 160 is connected to a second RF switch port 165. DPDT switch 140 is comprised of two single pole double throw (SPDT) switches, 170 and 180. The DPDT switch 140 has a transmit switch port 172 for supplying a signal to the switch 140 to be transmitted by the antennas 150,160 and a receive switch port 182 for providing a signal received from the antennas 150,160. SPDT switch 170 allows the connection either from the transmit switch port 172 to the first RF switch port 155 or the transmit switch port 172 to the second RF switch port 165. Similarly, SPDT switch 180 supports the connection either from the receive switch port 182 to the second RF switch port 165 or from the receive switch port 182 to the first RF switch port 155. Switching operations are enabled by 2 or 4 switch controls 190. Single switch control is also possible by using a parallel to series logic decoder.
FIG. 2 shows a circuit diagram of the conventional DPDT T/R switch 140, in which FETs (field effect transistors) are used to implement the switching functionality. A transmitter input port (Tx In) is coupled through a capacitor C1 to a drain terminal of a first FET Q1. A source terminal of Q1 is coupled through capacitor C2 to a first antenna (Ant1). “Tx In” is also coupled though capacitor C1 to a drain terminal of a second FET Q2. A source terminal of Q2 is coupled through capacitor C3 to a second antenna (Ant2). A receiver output port (Rx Out) is coupled through capacitor C4 to a drain terminal of a third FET Q3. A source terminal of Q3 is coupled to Ant1. “Rx Out” is also coupled through capacitor C4 to a drain terminal of a fourth FET Q4. A source terminal of Q4 is coupled to Ant2. A transmit enable to Ant1 (Tx to Ant1 Enable) is coupled to a gate of Q1 through resistor R1. A transmit enable to Ant2 (Tx to Ant2 Enable) is coupled to a gate of Q2 through resistor R2. A receive enable to antenna 1 (Rx to Ant1 Enable) is coupled to a gate of Q3 through resistor R3. A receive enable to Ant2 (Rx to Ant2 Enable) is coupled to a gate of Q4 through resistor R4. By setting the transmit and receive enables appropriately for the respective FETs, it is possible to send a transmit signal to one of the two antennas, obtain a receive signal from one of the two antennas or transmit to one of the two antennas and receive from the other of the two antennas simultaneously.
Due to current size demands placed on cellular handsets there are a limited number of effective locations that are possible for locating more than one antenna if trying to implement a DPDT switch in the WLAN architecture. In addition, insufficient spacing between antennas not only causes a high antenna coupling, but also reduces the effectiveness of spatial diversity.
Portable wireless devices such as cellular radios utilizing time division multiple access (TDMA) and frequency division duplexing (FDD) functionality, for example GSM (Global System for Mobile Communications) and TDMA, can employ timing coordination between different radio operations, which avoids interference between radios and provides pseudo simultaneous operation. However, such time coordination cannot be applied to CDMA and other code/frequency division multiple access cellular radios. A true simultaneous operation of multiple radios can only be realized with low interference between radios.